Batteries or cells, especially secondary batteries now draw a greater attention. They are used in a wider variety of applications including mobile phones, portable video cameras, portable computers, and electric automobiles. In particular, lithium ion secondary batteries are used in these applications because they are superior in voltage and energy density to traditionally available batteries using aqueous electrolytic solution such as Ni-MH batteries, Ni—Cd batteries, and H2SO4—Pb batteries. The lithium ion secondary batteries, however, suffer from safety problems of potential ignition or explosion because organic electrolyte solutions are used.
On the other hand, studies have been made on batteries not using electrolytic solution, that is, batteries using electrolytes in the form of ion-conductive solids. One exemplary electrolyte is polyethylene oxide having lithium perchlorate dissolved therein as described in U.S. Pat. No. 4,303,748 or JP-A 2-56870. This electrolyte, however, cannot be used in practice because of insufficient ionic conductivity and a high contact resistance with positive and negative electrodes. Nevertheless, since then, active efforts have been devoted for the research of ion-conductive polymers.
As compared with inorganic materials, organic materials have advantages including a low specific gravity, easy molding, and easy formation of flexible thin film. In general, solid electrolytes including solid polymer electrolytes (SPE) must meet some requirements, (1) moldability, (2) high ionic conductivity, and (3) safety, i.e., inertness to electrochemically active material. Importance is given in the order of (2), (3) and (1). The organic materials under investigation in the art are mostly polyethylene derivatives because of the course of research described above, and further include dielectrics, such as polyvinylidene fluoride and polyacrylonitrile having additives added thereto and exhibiting a conductivity σ of 10−7 to 10−5 S/cm. Although these prior art polymers were expected to be applied to batteries as organic solid electrolyte, they did not fully meet the above requirements to a sufficient extent to find practical battery and other applications. For example, polyethylene oxide derivatives have the problem of crystallization at low temperature. Most dielectrics are not regarded as having a high permittivity (or dielectric constant), as demonstrated by a relative permittivity of 9.2 for polyvinylidene fluoride and a relative permittivity of 8.0 for polyacrylonitrile. They do not contain a large amount of electrolyte, with a smaller number of carrier ions being available. Solid electrolytes having a high ionic conductivity were not available.
JP-A 4-363869 describes that dielectrics having cyano groups are applicable as the solid electrolyte. This patent discloses polymers in the form of polyvinyl alcohol, polysaccharides and derivatives thereof which are substituted with cyanoethyl groups. They exhibit a relatively high ionic conductivity in a system comprising lithium ions. U.S. Pat. No. 5,641,590 or JP-A 9-50824 discloses a gel electrolyte comprising a cyano-containing dielectric, which also exhibits a relatively high ionic conductivity in a system comprising lithium ions. These cyano-containing dielectrics satisfy requirement (1) to a full extent and requirement (2) to a certain extent, but requirement (3) of inertness to electrochemically active material to a short extent. These cyano-containing dielectrics are derived from base polymers having hydroxyl groups by substituting cyanoethyl groups for the hydroxyl groups. It is difficult at the state-of-the-art to substitute cyanoethyl groups for the hydroxyl groups entirely, with a certain proportion of hydroxyl groups being left behind. When these cyano-containing dielectrics are used in lithium ion secondary batteries, inevitably gas evolves probably due to alcoholate reaction of hydroxyl groups with lithium ions.
Further, JP-A 3-74419 describes radical copolymers of vinyl ethers and cyanoethyl acrylate of formula (1) or cyanoethyl methacrylate of formula (2), both shown later. It is difficult to produce copolymers having a sufficient degree of polymerization.